Process for plating metals



United States Patent 3,445,351 PROCESS FOR PLATING METALS Donald A.Swalheim and Robert W. Mackey, Wilmington, Del., assignors to E. I. duPout de Nemours and Company, Wilmington, Del., a corporation of DelawareFiled Oct. 21, 1964, Ser. No. 405,362 Int. Cl. C23b 5/52, 5/50, 5/14 US.Cl. 204-37 3 Claims ABSTRACT OF THE DISCLOSURE This invention relates tothe production of electrolytic tin plating having high corrosionresistance. More particularly, the invention relates to applying anultra-thin preplate deposit of nickel-tin or nickel onto a base metalsurface prior to plating the tin.

Recent efforts to improve corrosion resistance of tin plate to providewhat is referred to as Type K or Grade A plate have been directed mainlytoward melting or flowbrightening the tin layer. Embodiments of thistechnique are described in US. Patents 2,661,328 and 3,062,275. Animprovement in such a process is described in our copending applicationSwalheim & Mackey, 'Ser. No. 317,462, filed Oct. 21, 1963. In thisapplication the improved process comprises flow-brightening the tinplate by heating it at an elevated temperature and thereafter quenchingthe tin plate within a predetermined peroid of time. While the improvedprocess does provide high quality plate, it would be most desirable tohave greater flexibility in the use of processing conditions and, inparticular, to be able to reduce the melting temperature and hold-uptime between melting and quenching the tin plate in the flow-brighteningste I is, therefore, a primary object of this invention to provide animprovement in the process of tin plating which gives high quality plateusing a moderate temperature and a reduced delay time between heatingand quenching. Other objects will be apparent from the detaileddescription which follows hereinafter.

The objects of this invention are accomplished by our discovery that anultra-thin, i,e., from about 2X10 to 65 X 10- inch, coating ofnickel-tin alloy or nickel applied to the base metal prior to the tinplating provides substantial improvements in the quality of tin plateover a wide range of temperature and delay times. This coating orpreplate may be applied using conventional electroplating procedures.The necessity for controlling the thickness of the preplate within thelimits specified is graphically illustrated by the family of curvesshown on the accompanying drawing which will be discussed in detaillater herein.

In carrying out the process of our invention, the preplate may beapplied by using a conventional electroplating unit and one of the manyknown nickel or nickel-tin plating baths. Since a very thin coating isdesired, the current in the electroplating unit must be carefullycontrolled. Suitable apparatus such as a high current transistorswitching device capable of delivering a current pulse of controlledduration, e.g., 5 to 100 milliseconds, may be used. After application ofthe preplate, the sheet is then electroplated in the usual manner toprovide a tin plating of desired thickness, preferably 0.25 to 1.5pounds per base box. The plated sheet is then passed through a heaticeing device where it is heated to a temperature from about 540 to 850 F.,preferably 540 to about 700 F. The temperature is maintained for a veryshort period of time, e.g., up to 3.0 seconds and preferably 0.25 to 1.0second, after which the sheet is quenched, bringing the temperaturerapidly to a level of about 200 F. or lower.

The utility of this invention can be best demonstrated in terms of itsapplication. In succeeding paragraphs data are presented to show thatvery thin layers of nickel or nickel-tin alloy give substantialimprovements in corrosion resistance when the tin plating and flowbrightening operations are carried out in an otherwise normal manner. Asa further advantage, these improvements can be readily incorporated inhigh-speed electrotinning operations.

The corrosion resistance of tin plate is measured by the ATC (Alloy TinCouple) test. The results have been shown to correlate well with shelflife experience for tin cans packed with corrosive foods. In the ATCtest, the free tin layer is stripped from a test panel exposing theiron-tin alloy covering the steel base. The stripped test piece isconnected to a thin anode and placed in a cell where conditionssimulating those found inside sealed cans are maintained. After twenty(20) hours in this environment, the electric current flowing between thetest piece and the anode is measured and, when expressed as microamperesper square centimeter, is called the ATC value. Lower ATC valuesindicate better corrosion resistance. Type K or Grade A tin plate musthave an average ATC value less than 0.05 with of all tests less than0.085.

Referring to the drawing, the curves shown illustrate the effect ofthickness (d) of a nickel-tin preplate on the ATC value of a 0.75 poundper base box tin plate. The curves are grouped in families to show thata similar improvement in corrosion resistance (decrease in ATC value) isobtained at typical reflow temperatures (T) and delay times (0) withinthe ranges described above. A substantial improvement is obtained withvery thin preplates. Maximum improvement is obtained at thicknesses ofabout 65 X 10- inch and the effect diminishes as the thickness of thepreplate increases.

The data used in establishing the curves shown on the drawing wereobtained from a large number of experiments which will be described inthe following examples:

Example I Type L 220-finish low carbon steel of the type customarilyused for tin plate was used for preparation of samples. A 6" x 12" steelcathode strip was mounted on the rotor of a Du Pont rotating cathodecell which is described by Swalheim in Trans. Electrochem. Soc. 86, 395(1944) and, after conventional cleaning and pickling, the strip wasplated with a preplate of nickel-tin. The cathode was then removed fromthe preplating cell, rinsed in water and placed in another Du Pontrotating cathode cell to plate the tin. A tin plate of 0.75 pound perbase box was applied, using a conventional plating bath.

After plating, strips (2" x 6") were cut from the larger strips and wereheated to a carefully controlled given temperature by electricalresistance heating and, after a controlled, predetermined delay time,were dropped into a water quenching bath.

After quenching, the remaining tin plate was stripped from the samples,leaving the steel panels with an exposed iron-tin alloy coating. Thisstripping was done by immersing the panels in a 5% NaOH solution using aconstant voltage power supply to maintain a constant potential of 0.4volt between the panels and a stainless steel electrode. When all thefree tin was removed, the stripping current automatically diminished tozero. The stripped panels were then subjected to tests to determine ATC3 values in accordance with the test procedure previously described.

The nickel-tin alloy was plated from a bath having the followingcomposition:

Grams/liter SnCl 36 NaF 34 NaHF 16.9 NaCl 35 The pH of the bath wasadjusted to within the range of 2.0 to 2.5. The thickness of thenickel-tin was controlled by selecting the prescribed plating timerequired for passing of the coulombs to deposit the thickness ofnickel-tin alloy desired.

The nickel-tin alloy plating bath deposited an alloy containingapproximately 35% Ni and 65% Sn. The baths were operated at atemperature of 160 F. The results showing the effect of temperature anddelay time on ATC values using preplate deposits from the bath describedabove at thicknesses of 6.5, 36.5, 65.5, 200 and 455 x 10- inch,respectively, are recorded in the table which follows:

The results obtained, using various thicknesses of pieplate, are setforth in the following table:

The data illustrates similar advantages to those obtained in Example Iwhen using thin Ni-Sn preplates. A relatively dilute bath offers theadditional advantage from a commercial standpont of diminishing drag-outof ingredients.

Example JII A series of experiments was conducted in which nickelpreplates were applied to the steel substrates pior to ap- TABLE 1 ATCvalue (average) preplate thickness of- Delay time Melt temp.

(8%.) F.) 6.5)(10' 36.5)(10 65.5X 200x10 455x10- TABLE 2 [No Ni-SnPreplate] Melt temperature (F.) ATC value (avg) Delay time (sec.)

It will be noted from the foregoing table that Type K plate, i.e., platehaving an ATC value of less than 0.05 was not even produced at thereflow temperature of 595 C., with a relatively long delay time of 1.0second.

Example II The experiment described in Example I was repeated exceptthat the thin nickel-tin preplates were coated from a dilute bath havingthe following composition:

SnCl Grams/liter 3.6 NaF do 3.4 NaHF do 1.69 NaCl do 3.5 NiCl -6H O do-22.5

plying the tin plate. A nickel plating bath having the followingcomposition was used:

Panels 2" x 6" were cleaned and pickled in the conventional manner andwere then plated with nickel in a small laboratory cell underessentially static conditions. A very thin nickel preplate was obtainedby using equipment to provide a plating time in the order of 0.0025second and a current density during discharge of a set of condensers ofabout 400 amperes per square foot. After plating, the panels were platedwith 0.75 pound per base box of tin (45 10 inch) and were then flowbrightened at a temperature of about 605 F. Time of heating was 0.55second and the delay time between power shut-off and quenching in waterwas 0.1 second. The data obtained from the experiments are set forth inthe following table:

Example IV In another series of experiments, nickel preplates weredeposited by plating onto 6" x 12" steel panels using the DuPont rottingcathode cell. The preplated panels were then given a 0.075 pound perbase box tin plating, using the procedure described in Example IH. Thenickel plating bath used in the series of tests had the followingcomposition:

NiCl -6H O Grams/liter H BO d 7.5 P-4 Addition Agent ml./liter 3.5 pH3.5

Proprietary Additive Supplied by Harshaw Chemical Comparty forDecorative Nickel Plating.

The date obtained from the series of experiments are recorded in thetable which follows:

TABLE 5 Preplate thickness ATC value X 10- inch) (Average) None 0.279

scope thereof, it is to be understood that this invention is not to belimited to the specific embodiment thereof except as defined in theappended claims.

1. In a process for producing a bright tin coating on a base metal stripwhich comprises immersing said strip in an electrolytic cell containinga tin plating solution and thereafter flow brightening the resultingtincoated strip by heating the coated strip to a temperature from about540 to 850 F., the improvement comprising precoating the base metal inan electrolytic cell to provide an ultra-thin metal preplate having athickness in the range from about 2X10" to about X 10'- inch, saidpreplate being selected from the group consisting of nickel andnickel-tin alloy.

2. The process of claim 1 wherein said temperature is maintained for aperiod of time not greater than 3 seconds, after which the coated stripis rapidly quenched.

3. The process of claim 2 wherein said tin coating has a thickness fromabout 0.25 to about 1.5 pounds per base box.

References Cited UNITED STATES PATENTS 2,085,543 6/ 1937 Oplinger 204-172,266,330 12/1941 Nachtman 20428 2,303,035 1l/l942 Fink, 204-362,381,778 8/1945 Schoonmaker et a1 204-37 2,315,740 4/ 1943 Schoonmakeret a1. 204-37 3,260,580 7/1966 Kamm et al 29196.4 3,334,030 8/ 1967Notman 20437 3,285,838 11/1966 Morgan et a1 204-37 FOREIGN PATENTS297,161 9/1928 Great Britain.

473,479 10/ 1937 Great Britain.

484,909 5/ 1938 Great Britain.

JOHN H. MARK, Primary Examiner. W. B. VANSISE, Assistant Examimer.

US. Cl. X.R. 29196.4; 204-40

